It is certainly not unheard of for people on insulin to fall into a coma in their sleep and die. It almost happened to me just over 10 years ago. Luckily my husband woke to go to the loo, found me sweating & unconscious and called the paramedics. It was very frightening to realise that if it wasnt for him, I probably wouldn't be here.
I read somewhere that if you are unconscious for more than an hour with a hypo, you can be left brain damaged.
I'm always careful never to take short-acting too late at night, and always check my blood sugar before I go to sleep.
In answer to your question Martin, every hypo should be taken seriously, and dangerous if you don't treat it. I never leave home without glucose tablets or a bottle of lucozade; that's just common sense.
I once had a GP who asked me if I checked my own blood sugar at home, and many who've asked me if I have hypos! Diabetes Awareness campaigns are failing us type-1s, if even medics don't realise that hypos are part and parcel of being a diabetic on insulin. We all live with it, and learn how to deal with it, the dangerous part is mainly other people's ignorance.
My husband told me fairly recently of a story he read in the Telegraph of a man who was arrested because he was presumed drunk and disorderly. He was diabetic and died in police custody as a result of an unrecognised hypo.
Jus